Re: [PATCH 1/7] mm: introduce common struct mm_slot

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 29 Aug 2022 22:30:49 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote:

> At present, both THP and KSM module have similar structures
> mm_slot for organizing and recording the information required
> for scanning mm, and each defines the following exactly the
> same operation functions:
> 
>  - alloc_mm_slot
>  - free_mm_slot
>  - get_mm_slot
>  - insert_to_mm_slots_hash
> 
> In order to de-duplicate these codes, this patch introduces a
> common struct mm_slot, and subsequent patches will let THP and
> KSM to use it.

Seems like a good idea.

> --- /dev/null
> +++ b/mm/mm_slot.h
> @@ -0,0 +1,55 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#ifndef _LINUX_MM_SLOT_H
> +#define _LINUX_MM_SLOT_H
> +
> +#include <linux/hashtable.h>
> +#include <linux/slab.h>
> +
> +/*
> + * struct mm_slot - hash lookup from mm to mm_slot
> + * @hash: link to the mm_slots hash list
> + * @mm_node: link into the mm_slots list
> + * @mm: the mm that this information is valid for
> + */
> +struct mm_slot {
> +	struct hlist_node hash;
> +	struct list_head mm_node;
> +	struct mm_struct *mm;
> +};

It appears that the presence of an mm_struct in the hash list does not
contribute to the mm_struct's refcount?  That's somewhat unexpected.

It would be helpful to add some words here describing the means by
which a user of mm_slot would prevent the mm_struct from getting freed
while on the list.  I assume "caller must maintain a reference on the
mm_struct while it remains on an mm_slot hash list"?

> +#define mm_slot_entry(ptr, type, member) \
> +	container_of(ptr, type, member)
> +
> +static inline void *alloc_mm_slot(struct kmem_cache *cache)
> +{
> +	if (!cache)	/* initialization failed */
> +		return NULL;
> +	return kmem_cache_zalloc(cache, GFP_KERNEL);
> +}
> +
> +static inline void free_mm_slot(struct kmem_cache *cache, void *objp)
> +{
> +	kmem_cache_free(cache, objp);
> +}
> +
> +#define get_mm_slot(_hashtable, _mm)					       \
> +({									       \
> +	struct mm_slot *tmp_slot, *mm_slot = NULL;			       \
> +									       \
> +	hash_for_each_possible(_hashtable, tmp_slot, hash, (unsigned long)_mm) \
> +		if (_mm == tmp_slot->mm) {				       \
> +			mm_slot = tmp_slot;				       \
> +			break;						       \
> +		}							       \
> +									       \
> +	mm_slot;							       \
> +})

Is there a reason why this must be implemented as a macro?  That's
preferable, although this may be overly large for inlining.  mm/util.c
might suit.

> +#define insert_to_mm_slots_hash(_hashtable, _mm, _mm_slot)		       \
> +({									       \
> +	_mm_slot->mm = _mm;						       \
> +	hash_add(_hashtable, &_mm_slot->hash, (unsigned long)_mm);	       \
> +})

Does this need to be a macro?


And the naming.  Can we please have

mm_slot_entry
mm_slot_alloc
mm_slot_free
mm_slot_get
mm_slot_insert

Also, "get" usually implies that a refcout is taken on the obtained
object, so mm_slot_lookup() would be more appropriate.